Method of determining graphite shapes in nodular iron



NOV. 8, 1966 W` B, LARSON ETAL 3,284,192

METHOD OF DETERMINING GRAPHITE SHAPES IN NODULAR IRON Filed Jan. 22, 1963 ,u cra 5 Pff 35C. 4400 o /a a 30 40 fa @o raw .9a /oo 6MP/#Tf ATTORNEY United States Patent O 3,284,192 METHOD F DETERMINING GRAPHITE SHAIES IN NODULAR IRON William B. Larson, Kenneth E. Spray, and Thomas W. Mueller, Saginaw, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Jan. 22, 1963, Ser. No. 253,119 2 Claims. (Cl. 75--130) This invention relates to a method for evaluating the graphitic micro-structure of nodular iron.

The shape and distribution of carbon particles in cast iron influence its mechanical properties su-ch as strength and ductility. Therefore, for any given application it is desirable to regulate the shape land distribution of the graphite spheroids or nodules in the cast iron so that t-he resultant iron members will have optim-um properties for their intended use. It is particularly important to be ab'le to check what lthe final resultant cast iron is going to be from lany melt 'before th-at melt is poured so that corrections can still ybe made, if necessary.

lIt is a principal object -in making this invention to provide a method of evaluating what the graphitic microstructure of cast iron members from a melt will -be in its final solidified state While the melt is yet in the molten state so lthat corrections Imay still be made.

It is la further object in making this invention to provide a sample from a melt for testing, whose characteristics can then be used to determine the necessity of adding any ingredients to the melt to obtain desired end properties of castings therefrom.

It is a still further object in 'making this invention to provide a method of sampling a me-lt to determine the shape and distribution of spheroid graphite particles in the resulting cast iron while the melt is still in condition to have lits chemistry regula-ted.

With these and other objects in view which will become apparent as the specification proceeds, our invention will be best understood b-y reference to the following specification and `claims and lthe illustrations in the accompanying drawings, in which:

FIGURE 1 is a sketch showing a form of cast sample used for test-ing;

FIG. 2 is a schematic and block diagram of a system embodying our invention;

FIG. 3 is a number of sketches showing in enlarged form the various possible shapes taken b-y the graphite particles in iron; and,

FIG. 4 is a graph of frequency of reasonance of the sample part ploted against percent of spheroid particles contained therein.

Referring now more particularly to the drawings the primary purpose of the method is to extract a sample of the melt and to cast the same in a given form so that it can be easily set into vibration and its resonant frequency determined. The resonant frequency of the part is in turn determined by the micro-structure thereof. If all other variables except the shape or form of the graphite nodules are kept the same for all cast samples of iron then the shape or form of these nodules is the only thing which would tend .to vary the resonant frequency of the sample part and by measuring the same the shape of the particles can be determined from empirical charts and, therefore, the `mechanical characteristics of the iinal iron found. While various sample test shapes may be used, We have selected as one of the best a shape which resembles that of a tuning fork -as it can more readily be set into vibration. The configuration shown yat 2 in FIG. 1 may, therefore, be used and molds may be pivoted in which to cast samples of 4this configuration. After the shell molds have been made from patterns along the lines of the part 2, 'a sample of molten nodular iron from the 3,284,192 Patented Nov. 8, 1966 melt to be checked is poured into the mold. After the casting temperature has dropped to, for example, between 1900 and 1800 F. the mo'ld is broken open and the casting is quenched in oil at room temperature. The casting is left in the oil from 15 to 3VO seconds when it is removed and quenched in water bringing it to room temperature in about thirty seconds. By the use of this mounted on one side of the sample 2 which can be raised and when released descends to strike the tun-ing fork sample Iand set it into vibration. On .the opposite side of the sa-mple 2` there is mounted a microphone pick-up member 8 which is connected directly into a pre-amplitier 10 .and `thence to the vertical plates lof an oscillograph 12. The horizontal plates of the -oscillograph are connected to the output of an audio oscillator 14. The frequency of vibration `of the sample tuning fork yoke 2 is determined by adjusting the output Iof the audio oscillator 14 until a Lissajous figure is obtained on the screen of the oscillog-raph. The frequency is then read directly from the dial of the audio oscillator.

All parameters of each sample `are kept the same, such as size, shape, rates of cooling, etc. Therefore, the only variable 'between samples will be the size Iand number of the graphite nodules, the character of which can be determined by the resonant frequency of the device. Frequencies for different nodularized tuning fork sample yokes -a-re empirically determined and recorded and correlated to their graphite shapes through micro-analysis of the yokes. On this basis frequencies outside of a specific range indicate 4unacceptable -graphite shapes in the final casting 'and the need of adding corrective chemicals. The use o-f tuning fork yoke standardizes the size and shape of the sample, leaving `the material (size Iand shape of graphite nodules) as the only variable to affect the frequency. Nodular iron is ,a ferrous alloy of carbon, silicon and Imanganese with free graphite. Because the casting is quenched in the same manner each time a sample yoke is poured the alloy develops the same micro-structure martensite. Free graphite is then the main determinant of the frequency of vibration. This free graphite in nodular iron may exist in various shapes. The shape tof the graphite is important because it determines the mechanical properties ofthe resultant cast parts. The optimum 4graphite shape for the best mechanical properties is a perfect sphere. Deviations toward flake graphite result -in a reduction in mechanical properties. The highest frequencies of vibration are recorded when graphite is in the spheroidal form or lto state it another way in which the graphite occupies the least volume. As the graphite nodular shape deviates toward flakes the frequency of vibration decreases proportionately.

FIG. 3 shows the various forms that the nodular graphite might take. At A the nodules 16 are substantially spherical which is the best and most desired form. At B the nodules are not i-n spherical form but are still relatively round and compact. At C the nodule-s have begun to lengthen tout and be-come elongated so that the mechanical properties of `any iron in which. these appear would not be as satisfactory as those containing nodules of form-s A or B, and at D the nodules have now reached full ake form and the iron would probably be generally unacceptable for many uses.

The graph sof FIG. 4 plots resonant frequency of vibration of samples in cycles per second against percent of sphenical graphi-te nodules. This curve has been plotted from the results of a series of tests. Thus the cu-rve 18 shows that a sample casting with substantially 100% spherical nodules will vibrate in the range between 4300 `and 4400 cycles per second. Such a `casting would have satisfactory physical properties. As the amount of spheroid decreases and the amount lof nodular flakes increase `and the resonant frequency -of vibration of the sample decreases until at 30% spheroid the frequency has been lowered to some 4100 cycles. At this point the resultant iron would probably `be unacceptable -and it would be necessary to add to the melt some material such Ias magnesium, tellurium or some other `appropriate alloying element which would tend to cause the graphite to assume more spherical sha-pe in solidifying to pro-duce s'atisfgactorycastings. Actually .in tests which have been run, i-t tis found that in testing sam-ple parts from .a single heat that the frequency of vibration yof these parts did not va-ry over 46 cycles lor over 1.1%. Therefore, this is an excellent 'and :accurate method of testing the melt to determine the characteristics Iof the nal casting at a point where corrections can be made and proper qual-ity iron obtained under most circumstances.

1What is claimed is:

1. In la method `of testing molte-n free graphite containing ferrous metal to determine such nal physical properties of par-ts cast therefrom `as strength and ductility while corrective measures may be taken including the steps of casting a sample tof a given shape Iand size, reducing the temperature of the sample casting on a de- -ned schedule to room temperature, setting said sample casting into resonant vibration, determining the frequency of said resonant vibration as an indication of the sizetand shape of the graphite particles therein which deter-mines the physical properties of parts cast from the melt and adding corrective materials to the melt if necessary to produce .parts yof desired strength from the melt.

Z. In a method of testing -a metallic me-lt of free graphite containing ferrous metal to determine the physical characteristics such as strength `and Iductility of parts cast therefrom, the steps of casting `a sample of .a given size and shape, cooling the sample to room temperature ion a dened schedule, setting the sample into resonant vibration, determining the frequency of such Vibration, comparing the resonant frequency with `a known resonant frequency obtained by .an identical test of -an identical sample containing known size and shape yof graphite particles, `and adjusting the com-position and cooling rate of the melt to produce cast iron wi-th substantially idenical properties as those of said ident-ical sample.

Fuller, A. G.: BCIRA Journal, volume l0, 1962, page 363.

McMaster, R. C.: Nondestructive Testing Handbook, volume II, chapter 51. The Ronald Press Co., 1959.

DAVID L. RECK, Primary Examiner.

H. W. TARRING, Assistant Examiner.

UNITED STATES PATENT oFFICE CERTIFICATE OF CORRECTION Patent No. 3,284,192 November 8, 1966 William B. Larson et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column l, line 69, for "pivoted" read provided Signed and sealed this 12th day of September 1967.

(SEAL) Attest:

EDWARD J. BRENNER ERNEST W. SWIDER Commissioner of Patents Attesting Officer 

1. IN A METHOD OF TESTING MOLTEN FREE GRAPHITE CONTAINING FERROUS METAL TO DETERMINED SUCH FINAL PHYSICAL PROPERTIES OF PARTS CAST THERFROM AS STRENGTH AND DUCTILITY WHILE CORRECTIVE MEASURE MAY BE TAKEN INCLUDING THE STEPS OF CASTING A SAMPLE OF A GIVEN SHAPE AND SIZE, REDUCING THE TEMPRATURE OF THE SAMPLE CASTING ON A DEFINED SCHEDULE TO ROOM TEMPERATURE, SETTING SAID SAMPLE CASTING INTO RESONANT VIBRATION, DETERMINING THE FREQUENCY OF SAID RESONANT VIBRATION AS AN INDICATION OF THE SIZE AND SHAPE OF THE GRAPHITE PARTICLES THEREIN WHICH DETERMINES 